Lean-burning engines, or engines that run on an air/fuel mixture with a stoichiometrically greater amount of air than fuel, can offer improved fuel economy relative to engines configured to run on stoichiometric air/fuel mixtures.
However, lean-burning engines also may pose various disadvantages. For example, burning a lean air/fuel mixture may decrease the reduction of nitrogen oxides (collectively referred to as “NOx”).
Various mechanisms have therefore been developed to reduce NOx emissions in lean-burning engines. One mechanism is a catalyst known as a lean NOx trap. The NOx trap is a catalytic device typically positioned downstream of the catalytic converter in an emissions system, and is configured to retain NOx when the engine is producing a lean exhaust for eventual reduction when the engine produces a rich exhaust. A typical NOx trap includes an alkaline-earth metal, such as barium, and/or an alkali metal, such as potassium, to which NOx adsorbs when the engine is running a lean air/fuel mixture. The engine can then be configured to produce a rich exhaust containing carbon monoxide, hydrogen gas and various hydrocarbons to reduce the NOx in the trap, thus decreasing NOx emissions and purging the trap.
Various methods may be used to determine when a NOx trap needs to be purged. For example, a NOx sensor may be used to measure NOx emissions behind a NOx trap during lean engine operation. With an estimate of the feedgas NOx concentration (which is the NOx contained in exhaust from the engine), the NOx storage efficiency of the trap can be calculated. This may be used to determine when the storage efficiency of the trap has dropped below a minimum threshold value, and therefore when the trap needs to be purged. However, NOx sensors typically are expensive, and can significantly increase the cost of the lean aftertreatment system.
The inventors herein have realized that a NOx storage capacity of a catalytic device such as a NOx trap may be efficiently and accurately estimated from outputs of exhaust oxygen sensors positioned upstream and downstream of the catalytic device by utilizing a method of operating an engine, wherein one embodiment of the method includes operating the engine at a rich air/fuel ratio for a first interval; adjusting a temperature of the catalytic device to a diagnostic temperature for measuring an oxygen uptake by the catalytic device; operating the engine at a lean air/fuel ratio for a second interval; and adjusting the temperature of the catalytic device to an operating temperature. An oxygen storage capacity of the catalytic device may be measured while at the diagnostic temperature from a difference between the signals from the upstream and downstream oxygen sensors, and a NOx storage capacity of the catalytic device may be determined from the measured oxygen storage capacity. By performing the diagnostic in the diagnostic temperature range, a more repeatable and robust OSC measurement can be made.
Also, note that a relative air/fuel ratio may be relative to stoichiometry, also referred to as a lambda. Lambda may be a mass ratio of air to fuel for a stoichiometric mixture divided by a mass ratio of air to fuel for the actual mixture being used. Lambda of 1 indicates a stoichiometric mixture; lambda greater than 1 is lean and lambda less than 1 is rich.